Positioning systems have been developed which are based on cellular mobile communication networks and in which a wireless terminal uses base station signal propagation times (differences in the time of arrival) for their positioning. In such OTDOA systems (observed time difference of arrival), the executability and precision of positioning are affected, among other things, by the number of base stations transmitting a signal whose propagation times the wireless terminal is capable of measuring. For the positioning, at least three different base stations are required to transmit a signal whose propagation time from the base station to the wireless terminal is known. However, the signal from some base stations may be too weak for the wireless terminal to take measurements with a sufficient reliability. Furthermore, in mobile communication systems based on the CDMA (code division multiple access) technique, each base station transmits at the same frequency. Thus, the signal of the serving base station may be so strong that it makes it more difficult to receive the signals of other base stations. In such a case, positioning may be even impossible. The serving base station refers to the base station, through which the wireless terminal communicates with the mobile communication network at a given time.
In so-called third generation mobile communication systems based on the spread spectrum technique, the base stations transmit a spread spectrum modulated signal, utilizing one or more spreading codes in the modulation. These systems apply the code division multiple access technique which makes it possible for several wireless terminals to communicate with the mobile communication network simultaneously. For such a system, the abbreviation CDMA is used, or WCDMA in the case of a wideband spread spectrum system. The base stations of the mobile communication system may be either synchronized, wherein the transmissions of the base stations are synchronized with each other, or non-synchronized, wherein each base station schedules its transmissions substantially independently of other base stations.
The signal transmitted from the base station can also propagate otherwise than along the line of sight, particularly when there are obstacles affecting the propagation of the signal and/or objects reflecting the signal between the base station and the wireless terminal. In particular, ground topography and buildings may cause attenuation and reflections in the signal. The same signal can thus come to the receiver along more than one route, which is called multipath propagation. In multipath propagation, the signals do not necessary travel along the same path, wherein the signal travelling along different paths comes to the receiver at different times, causing more than one correlation peak in the correlator of the receiver. Furthermore, the signal travelled along the line of sight may be attenuated so much that it is not detected by the receiver at all. Thus, the first correlation peak does not correspond to the shortest possible distance but the path travelled by a signal of multipath propagation. The incorrect distance data will cause positioning errors in systems which apply the time of flight of the signal transmitted from base stations to the wireless terminal and the known position of the base station, for positioning of the wireless terminal.
To receive the signal transmitted by the base station, the wireless terminal must perform acquisition to this signal. This can be implemented, for example, in such a way that the base station regularly transmits a primary/secondary synchronization code which is known to the receiver of the wireless terminal. Thus, the receiver of the wireless terminal determines, for example on the basis of cross-correlation, the code phase of the transmission and, on the basis of this code phase data, adjusts its own receiver to the correct phase and starts to receive the signal. On the other hand, it is possible to transmit, substantially continuously, a synchronizing signal on a given channel (so-called pilot channel), which signal is modulated by a scrambling code. This scrambling code has a constant length and is repeated at regular intervals. The receiver can try to determine the code phase of this scrambling code and then perform acquisition.
In a prior art solution, the channel coding applies a so-called primary synchronization code whose length is 256 chips, as well as a secondary synchronization code whose length is also 256 chips. Each base station transmits the same primary synchronization code. The secondary synchronization code is preferably formed by selecting, from a given number of synchronization codes, a set of synchronization codes in a given sequence, wherein this sequence of synchronization codes forms a kind of a code word. For example, there are 16 synchronization codes and these synchronization codes are sequenced e.g. in the order of 1st, 5th, 3rd, 16th, 15th, 2nd, etc., to a code with a given length (e.g. 256 chips). The sequence of the selected codes can vary at different base stations. The channel coding is then followed by modulation with the scrambling code. This code is selected from a given number (e.g. eight) of scrambling codes in such a way that a given set of scrambling codes corresponds to a given code word of the secondary synchronization code. For example, on the pilot channel of the UMTS system, it is possible to use a total of 512 different codes. These codes are divided into 64 different sets of 8 codes each. The code word formed by the secondary synchronization codes indicates which of these 64 sets is in question. Within the set, the correct code can be found, for example, by trying all the eight ones in turn or in parallel and by selecting the code which best correlates with the received signal. Thus, on the basis of this scrambling code, it is possible to differentiate one base station from another. When transmission diversity is applied, the same base station can use a different scrambling code in each transmission sector, wherein one sector of the same base station can be differentiated from another in a corresponding way. In this system, the code phase is determined on the basis of the scrambling code. In such an arrangement, it is not easy to find weak signals, because the correlation length is limited by the length of the synchronization code which is only 256 bits.
An alternative presented in a prior art solution to eliminate the problem of the short code is to perform the synchronization on the pilot channel. In this solution, the pilot channel applies a code which consists of 38,400 chips and which is repeated at intervals of about 10 ms. The interval of the chips is about 260 ns. It is thus possible to achieve a processing gain of about 24 dB. A problem here is, for example, the fact that if the base stations are not synchronized with each other, the receiver of the wireless terminal does not have information about the correct code phase. In this case, the receiver of the wireless terminal must find out the correct code phase from a total of 38,400 different alternatives. In the worst case, this means scanning of all the code phases. This will consume a lot of power and take as long as about 1.5 s for each receiving channel. Furthermore, incorrect correlation peaks may cause an incorrect code phase interpretation which will lead to incorrect positioning.
International patent application WO 99/11086 presents a positioning system which determines the relative time differences (RTD) of signals from base stations in a reference mobile station. The positions of the reference mobile station and the base stations are known. In the user's mobile station, whose position is to be determined, the observed time differences (OTD) of transmissions from the respective base stations are recorded, wherein by comparing the time differences of transmissions from the base stations, measured by the reference mobile station and by the user's mobile station, it is possible to find out the position of the user's mobile station by utilizing the known position data. However, this system does not present means to improve the reception of a weak signal, wherein only such base stations can be used for positioning, whose signal is sufficiently strong both in the reference mobile station and in the user's mobile station. Furthermore, the system requires the use of a reference mobile station.